In this fanfiction, perhaps there's a spell gone wrong that transports Harry and his friends to a prehistoric world full of velociraptors. They have to use their magic skills to survive in this new environment. Harry might use his Patronus charm to fend off the raptors at first, but then they gradually learn to communicate with the raptors using some kind of translation spell and together they try to find a way back to their own time.
First, do your research on both Harry Potter elements and velociraptors. Know the rules of magic in the Harry Potter world and the behavior of velociraptors. Then, come up with an interesting plot. It could start with a mystery, like why are velociraptors suddenly appearing in Hogwarts? Next, develop your characters. How will Harry and his friends react to these strange creatures? And finally, write in a way that captures the essence of both the Harry Potter series and the uniqueness of velociraptors.
First, research both Harry Potter lore and velociraptors. Know the rules of animagus in the Harry Potter world. Then, create a character who has a reason to be a velociraptor animagus. For example, they could have a family curse or a unique magical ability that leads to this form.
One great 'Harry Potter Velociraptor Animagus' fanfiction is 'The Velociraptor in the Wizarding World'. It has an interesting plot where the main character is an animagus who turns into a velociraptor and gets involved in Hogwarts' mysteries.
Another concept could be that Harry Potter discovers he has a connection to Magneto's mutant powers. This might happen during a strange encounter with a dark magic object that unlocks a latent power within him. He then has to learn how to control these new abilities while also dealing with the threats in his own wizarding world, and he might seek out Magneto for guidance in a secret meeting across the boundaries of their two worlds.
One idea could be that Hawkgirl somehow gets transported to the Harry Potter universe. Maybe she arrives during a Quidditch match and her flying skills astound everyone. She could then join forces with Harry and his friends to fight against the dark forces in Hogwarts, using her Nth metal mace and flight abilities in unique ways.
One idea could be a crossover where Harry Potter accidentally ends up in the world of Ranma 1/2. Maybe he's on a magical experiment gone wrong and lands in Nerima. He could use his magic to help Ranma deal with his curse, like creating a spell to keep him from changing into a girl when splashed with cold water.
One idea could be a crossover where Harry Potter accidentally travels to the One Piece world and meets Whitebeard. He could learn about their different forms of magic and power, and together they face a new threat that combines the dark magic from his world and the powerful pirates in the One Piece world.
One idea could be that Fluttershy somehow gets transported into the Harry Potter world. Maybe she arrives at Hogwarts as a new kind of magical creature that the students have never seen before. Her gentle nature might initially cause confusion among the more rowdy students, but she could gradually make friends with the likes of Luna Lovegood, who would be intrigued by her uniqueness.
One idea could be that Gandalf somehow gets transported to the Harry Potter universe. He could become a mentor to Harry, using his vast magical knowledge and wisdom from Middle - earth. Maybe he helps Harry understand some ancient magic that even Dumbledore wasn't fully aware of.
One idea could be a crossover where Kakashi is summoned to the Harry Potter world by a dark magic spell gone wrong. He then uses his ninja skills and Sharingan to navigate the wizarding world, perhaps even teaching some Defense Against the Dark Arts classes with his unique combat knowledge.